Peer-Review Publications


L. J. Vormawah, M. Vilén, R. Beerwerth, P. Campbell, B. Cheal, A. Dicker, T. Eronen, S. Fritzsche, S. Geldhof, A. Jokinen, S. Kelly, I. D. Moore, M. Reponen, S. Rinta-Antila, S. O. Stock, and A. Voss
Isotope shifts from collinear laser spectroscopy of doubly charged yttrium isotopes
Phys. Rev. A, 97 :042504 (April 2018)
Collinear laser spectroscopy has been performed on doubly charged ions of radioactive yttrium in order to study the isotope shifts of the 294.6-nm 5s2S1/2→5p2P1/2 line. The potential of such an alkali-metal-like transition to improve the reliability of atomic-field-shift and mass-shift factor calculations, and hence the extraction of nuclear mean-square radii, is discussed. Production of yttrium ion beams for such studies is available at the IGISOL IV Accelerator Laboratory, Jyväskylä, Finland. This newly recommissioned facility is described here in relation to the on-line study of accelerator-produced short-lived isotopes using collinear laser spectroscopy and application of the technique to doubly charged ions.
V. Shevelko, Yu. A. Litvinov, T. Stöhlker, and I. Yu. Tolstikhina
Lifetimes of relativistic heavy-ion beams in the High Energy Storage Ring of FAIR
Nucl. Instr. Meth. Phys. Res. B, 421 :45 (April 2018)
The High Energy Storage Ring, HESR, will be constructed at the Facility for Antiproton and Ion Research, FAIR, Darmstadt. For the first time, it will be possible to perform experiments with cooled high-intensity stable and radioactive heavy ions at highly relativistic energies. To design experiments at the HESR, realistic estimations of beam lifetimes are indispensable. Here we report calculated cross sections and lifetimes for typical U88+, U90+, U92+, Sn49+ and Sn50+ ions in the energy range E = 400 MeV/u–5 GeV/u, relevant for the HESR. Interactions with the residual gas and with internal gas-jet targets are also considered.
H. Gies, F. Karbstein, C. Kohlfürst, and N. Seegert
Photon-photon scattering at the high-intensity frontier
Phys. Rev. D, 97 :076002 (April 2018)
The tremendous progress in high-intensity laser technology and the establishment of dedicated high-field laboratories in recent years have paved the way towards a first observation of quantum vacuum nonlinearities at the high-intensity frontier. We advocate a particularly prospective scenario, where three synchronized high-intensity laser pulses are brought into collision, giving rise to signal photons, whose frequency and propagation direction differ from the driving laser pulses, thus providing various means to achieve an excellent signal to background separation. Based on the theoretical concept of vacuum emission, we employ an efficient numerical algorithm which allows us to model the collision of focused high-intensity laser pulses in unprecedented detail. We provide accurate predictions for the numbers of signal photons accessible in experiment. Our study is the first to predict the precise angular spread of the signal photons, and paves the way for a first verification of quantum vacuum nonlinearity in a well-controlled laboratory experiment at one of the many high-intensity laser facilities currently coming online.
M. Reichert, A. Eichhorn, H. Gies, J. M. Pawlowski, T. Plehn, and M. M. Scherer
Probing baryogenesis through the Higgs boson self-coupling
Phys. Rev. D, 97 :075008 (April 2018)
The link between a modified Higgs self-coupling and the strong first-order phase transition necessary for baryogenesis is well explored for polynomial extensions of the Higgs potential. We broaden this argument beyond leading polynomial expansions of the Higgs potential to higher polynomial terms and to nonpolynomial Higgs potentials. For our quantitative analysis we resort to the functional renormalization group, which allows us to evolve the full Higgs potential to higher scales and finite temperature. In all cases we find that a strong first-order phase transition manifests itself in an enhancement of the Higgs self-coupling by at least 50%, implying that such modified Higgs potentials should be accessible at the LHC.
C. Jauregui, C. Stihler, A. Tünnermann, and J. Limpert
Pump-modulation-induced beam stabilization in high-power fiber laser systems above the mode instability threshold
Opt. Express, 26 :10691 (April 2018)
A new way of stabilizing the output beam of a fiber laser system operating above the mode instability threshold is described and the first proof-of-principle experimental results are presented. This technique, which relies on a modulation of the pump power, works by washing the thermally-induced refractive index grating out, which weakens the coupling efficiency between transverse modes. One of the main advantages of this simple, yet powerful, approach is that it can be easily incorporated in already existing fiber laser systems since it does not require any additional optical elements. Using this beam stabilization strategy, a significant pointing stability and beam quality improvement has been demonstrated up to an average power of \~600W, which is a factor of 2 above the mode instability threshold.
G. A. Becker, S. Tietze, S. Keppler, J. Reislöhner, J. H. Bin, L. Bock, F.-E. Brack, J. Hein, M. Hellwing, P. Hilz, M. Hornung, A. Kessler, S. D. Kraft, S. Kuschel, H. Liebetrau, W. Ma, J. Polz, H.-P. Schlenvoigt, F. Schorcht, M. B. Schwab, A. Seidel, K. Zeil, U. Schramm, M. Zepf, J. Schreiber, S. Rykovanov, and M.C. Kaluza
Ring-like spatial distribution of laser accelerated protons in the ultra-high-contrast TNSA-regime
Plasma Phys. Contr. F., 60 :055010 (April 2018)
The spatial distribution of protons accelerated from submicron-thick plastic foil targets using multi-terawatt, frequency-doubled laser pulses with ultra-high temporal contrast has been investigated experimentally. A very stable, ring-like beam profile of the accelerated protons, oriented around the target’s normal direction has been observed. The ring’s opening angle has been found to decrease with increasing foil thicknesses. Two-dimensional particle-in-cell simulations reproduce our results indicating that the ring is formed during the expansion of the proton density distribution into the vacuum as described by the mechanism of target-normal sheath acceleration. Here—in addition to the longitudinal electric fields responsible for the forward acceleration of the protons—a lateral charge separation leads to transverse field components accelerating the protons in the lateral direction.
A. Klenke, M. Müller, H. Stark, A. Tünnermann, and J. Limpert
Sequential phase locking scheme for a filled aperture intensity coherent combination of beam arrays
Opt. Express, 26 :12072 (April 2018)
We present a novel phase locking scheme for the coherent combination of beam arrays in the filled aperture configuration. Employing a phase dithering mechanism for the different beams similar to LOCSET, dithering frequencies for sequential combination steps are reused. By applying an additional phase alternating scheme, this allows for the use of standard synchronized multichannel lock-in electronics for phase locking a large number of channels even when the frequency bandwidth of the employed phase actuators is limited.
W. Paufler, B. Böning, and S. Fritzsche
Strong-field ionization with twisted laser pulses
Phys. Rev. A, 97 :043418 (April 2018)
We apply quantum trajectory Monte Carlo computations in order to model strong-field ionization of atoms by twisted Bessel pulses and calculate photoelectron momentum distributions (PEMD). Since Bessel beams can be considered as an infinite superposition of circularly polarized plane waves with the same helicity, whose wave vectors lie on a cone, we compared the PEMD of such Bessel pulses to those of a circularly polarized pulse. We focus on the momentum distributions in propagation direction of the pulse and show how these momentum distributions are affected by experimental accessible parameters, such as the opening angle of the beam or the impact parameter of the atom with regard to the beam axis. In particular, we show that we can find higher momenta of the photoelectrons, if the opening angle is increased.
E. Gelfer, N. Elkina, and A. Fedotov
Unexpected impact of radiation friction: enhancing production of longitudinal plasma waves
Sci. Rep., 8 :6478 (April 2018)
We study the penetration of ultra-intense (intensity I ≃ 10^(23–24) W/cm2) circularly polarized laser pulses into a thick subcritical plasma layer with accounting for radiation friction. We show that radiation pressure is enhanced due to radiation friction in the direction transverse to the laser pulse propagation, and that for stronger and longer laser pulses this mechanism dominates over the ordinary ponderomotive pressure, thus resulting in a substantionaly stronger charge separation than anticipated previously. We give estimates of the effect and compare them with the results of one and two dimensional particle-in-cell simulations. This effect can be important for laser-based acceleration schemes.
Y. Lin, Y. Yuan, F. Fang, and Z. Tan
A study of electric field distribution in Benjamin type proportional counter using finite element method
Appl. Radiat. Isot., 135 :142 (March 2018)
Tissue equivalent proportional counters (TEPCs) are commonly based on the Benjamin type of concept. Initially the electric field is optimized by pulse height measurement methods and only one optimum solution was established at that time. In this paper, the electric field distribution is analyzed and optimized using a three-dimensional finite element method. The calculations show that the characteristics of the radial electric field distribution of this type of counters can be equated to cylindrical counters using a pair of appropriate field shaping electrode. Furthermore, the paper analyzes the axial electric field distribution and the possibility of achieving a uniform electric field along its anode while reducing the size of Benjamin type proportional counter design down to 1/10 of currently feasible values with respect to the thinnest available anode wire diameters.
A. Blinne, D. Schinkel, S. Kuschel, N. Elkina, S. G. Rykovanov, and M. Zepf
A systematic approach to numerical dispersion in Maxwell solvers
Comput. Phys. Commun., 224 :273 (March 2018)
The finite-difference time-domain (FDTD) method is a well established method for solving the time evolution of Maxwell’s equations. Unfortunately the scheme introduces numerical dispersion and therefore phase and group velocities which deviate from the correct values. The solution to Maxwell’s equations in more than one dimension results in non-physical predictions such as numerical dispersion or numerical Cherenkov radiation emitted by a relativistic electron beam propagating in vacuum. Improved solvers, which keep the staggered Yee-type grid for electric and magnetic fields, generally modify the spatial derivative operator in the Maxwell–Faraday equation by increasing the computational stencil. These modified solvers can be characterized by different sets of coefficients, leading to different dispersion properties. In this work we introduce a norm function to rewrite the choice of coefficients into a minimization problem. We solve this problem numerically and show that the minimization procedure leads to phase and group velocities that are considerably closer to c as compared to schemes with manually set coefficients available in the literature. Depending on a specific problem at hand (e.g. electron beam propagation in plasma, high-order harmonic generation from plasma surfaces, etc.), the norm function can be chosen accordingly, for example, to minimize the numerical dispersion in a certain given propagation direction. Particle-in-cell simulations of an electron beam propagating in vacuum using our solver are provided.
S. H. Hendi, B. Eslam Panah, and S. Panahiyan
Black Hole Solutions in Gauss-Bonnet-Massive Gravity in the Presence of Power-Maxwell Field
Fortschr. Phys., 66 :1800005 (March 2018)
Motivated by recent progresses in the field of massive gravity, the paper at hand investigates the thermodynamical structure of black holes with three specific generalizations: i) Gauss-Bonnet gravity which is motivated from string theory ii) PMI nonlinear electromagnetic field which is motivated from perspective of the QED correction iii) massive gravity which is motivated by obtaining the modification of standard general relativity. The exact solutions of this setup are extracted which are interpreted as black holes. In addition, thermodynamical quantities of the solutions are calculated and their critical behavior are studied. It will be shown that although massive and Gauss-Bonnet gravities are both generalizations in gravitational sector, they show opposing effects regarding the critical behavior of the black holes. Furthermore, a periodic effect on number of the phase transition is reported for variation of the nonlinearity parameter and it will be shown that for super charged black holes, system is restricted in a manner that prevents it to reach the critical point and to acquire phase transition. In addition, the effects of geometrical structure on thermodynamical phase transition will be highlighted.
O. Novak, R. Kholodov, A. Surzhykov, A. N. Artemyev, and T. Stöhlker
K-shell ionization of heavy hydrogenlike ions
Phys. Rev. A, 97 :032518 (March 2018)
A theoretical study of the K-shell ionization of hydrogenlike ions, colliding with bare nuclei, is performed within the framework of the time-dependent Dirac equation. Special emphasis is placed on the ionization probability that is investigated as a function of impact parameter, collision energy, and nuclear charge. To evaluate this probability in a wide range of collisional parameters we propose a simple analytical expression for the transition amplitude. This expression contains three fitting parameters that are determined from the numerical calculations, based on the adiabatic approximation. In contrast to previous studies, our analytical expression for the transition amplitude and ionization probability accounts for the full multipole expansion of the two-center potential and allows accurate description of nonsymmetric collisions of nuclei with different atomic numbers Z1≠Z2. The calculations performed for both symmetric and asymmetric collisions indicate that the ionization probability is reduced when the difference between the atomic numbers of ions increases.
A. M. Sayler, E. Eckner, J. McKenna, B. D. Esry, K. D. Carnes, I. Ben-Itzhak, and G.G. Paulus
Nonunique and nonuniform mapping in few-body Coulomb-explosion imaging
Phys. Rev. A, 97 :033412 (March 2018)
Much of our knowledge of molecular geometry and interaction dynamics comes from indirect measurements of the molecular fragments following breakup. This technique—Coulomb-explosion imaging (CEI), i.e., determining the initial molecular configuration of a system from the momenta of the resulting fragments using knowledge of the particle interactions—is one of the fundamental tools of molecular physics. Moreover, CEI has been a staple of molecular studies for decades. Here we show that one often cannot assign a unique initial configuration to the few-body breakup of a polyatomic molecule given the measurement of the resulting fragments' momenta. Specifically, multiple initial configurations can result in identical momenta for a molecule breaking into three or more parts. Further, the nonunique and nonuniform mapping from the initial configuration to the measured momenta also significantly complicates the determination of molecular alignment at the time of breakup.
A. K. Arunachalam, M. B. Schwab, A. Sävert, and M.C. Kaluza
Observation of non-symmetric side-scattering during high-intensity laser-plasma interactions
New J. Phys., 20 :033027 (March 2018)
Non-symmetric side-scattering has been observed during the interaction between a high-intensity laser pulse and under-dense argon plasma. The angle between the laser's forward direction and the scattered radiation is found to decrease for increasing electron densities ranging from 0.01 to 0.25n c , where n c is the critical density for the laser wavelength. We show that the observed features of the scattering cannot be described by Raman side-scattering but can be explained to be a consequence of the non-uniform density distribution of the plasma with the scattering angle being oriented along the direction of the resulting electron density gradient.
L. Obst, J. Metzkes-Ng, S. Bock, G. E. Cochran, T. E. Cowan, T. Oksenhendler, P. L. Poole, I. Prencipe, M. Rehwald, C. Rödel, H.-P. Schlenvoigt, U. Schramm, D. W. Schumacher, T. Ziegler, and K. Zeil
On-shot characterization of single plasma mirror temporal contrast improvement
Plasma Phys. Contr. F., 60 :054007 (March 2018)
We report on the setup and commissioning of a compact recollimating single plasma mirror (PM) for temporal contrast enhancement at the Draco 150 TW laser during laser-proton acceleration experiments. The temporal contrast with and without PM is characterized single-shot by means of self-referenced spectral interferometry with extended time excursion at unprecedented dynamic and temporal range. This allows for the first single-shot measurement of the PM trigger point, which is interesting for the quantitative investigation of the complex pre-plasma formation process at the surface of the target used for proton acceleration. As a demonstration of high contrast laser plasma interaction we present proton acceleration results with ultra-thin liquid crystal targets of ~ 1 μm down to 10 nm thickness. Focus scans of different target thicknesses show that highest proton energies are reached for the thinnest targets at best focus. This indicates that the contrast enhancement is effective such that the acceleration process is not limited by target pre-expansion induced by laser light preceding the main laser pulse.
F. Karbstein, and E. A. Mosman
Photon polarization tensor in circularly polarized Hermite- and Laguerre-Gaussian beams
Mod. Phys. Lett. A, 33 :1850044 (March 2018)
We derive analytical expressions for the photon polarization tensor in circularly polarized Hermite-Gaussian (HG) and Laguerre-Gaussian (LG) beams, complementing the corresponding results for linearly polarized beams obtained recently. As they are based upon a locally constant field approximation of the one-loop Heisenberg–Euler effective Lagrangian for quantum electrodynamics (QED) in constant fields, our results are generically limited to slowly varying electromagnetic fields, varying on spatial (temporal) scales much larger than the Compton wavelength (time) of the electron.
M. Taylor, M. Coughlan, G. Nersisyan, L. Senje, D. Jung, F. Currell, D. Riley, C. L. S. Lewis, M. Zepf, and B. Dromey
Probing ultrafast proton induced dynamics in transparent dielectrics
Plasma Phys. Contr. F., 60 :054004 (March 2018)
A scheme has been developed permitting the spatial and temporal characterisation of ultrafast dynamics induced by laser driven proton bursts in transparent dielectrics. Advantage is taken of the high degree of synchronicity between the proton bursts generated during laser-foil target interactions and the probing laser to provide the basis for streaking of the dynamics. Relaxation times of electrons (<10⁻¹² s) are measured following swift excitation across the optical band gap for various glass samples. A temporal resolution of <500 fs is achieved demonstrating that these ultrafast dynamics can be characterized on a single-shot basis.
A. Adelmann, B. Hermann, R. Ischebeck, M. Kaluza, U. Locans, N. Sauerwein, and R. Tarkeshian
Real-Time Tomography of Gas-Jets with a Wollaston Interferometer
Appl. Sci., 8 :443 (March 2018)
A tomographic gas-density diagnostic using a Single-Beam Wollaston Interferometer able to characterize non-symmetric density distributions in gas jets is presented. A real-time tomographic algorithm is able to reconstruct three-dimensional density distributions. A Maximum Likelihood-Expectation Maximization algorithm, an iterative method with good convergence properties compared to simple back projection, is used. With the use of graphical processing units, real-time computation and high resolution are achieved. Two different gas jets are characterized: a kHz, piezo-driven jet for lower densities and a solenoid valve-based jet producing higher densities. While the first jet is used for free electron laser photon beam characterization, the second jet is used in laser wake field acceleration experiments. In this latter application, well-tailored and non-symmetric density distributions produced by a supersonic shock front generated by a razor blade inserted laterally to the gas flow, which breaks cylindrical symmetry, need to be characterized.
C. Jauregui, F. Stutzki, A. Tünnermann, and J. Limpert
Thermal analysis of Yb-doped high-power fiber amplifiers with Al:P co-doped cores
Opt. Express, 26 :7614 (March 2018)
It has been recently shown that photodarkening can significantly reduce the mode instability threshold in high power Yb-doped fiber amplifiers, thus resulting in an even more severe limitation to the scaling of the output average power of these systems. Therefore, an efficient reduction of photodarkening in an Yb-doped active fiber will lead to very significant gains in the output average power delivered by such systems. In this context, it has been reported that photodarkening can be significantly mitigated when co-doping a fiber core with Al and P, which makes this approach potentially appealing to increase the TMI threshold. Unfortunately co-doping the fiber core with Al and P also alters the effective cross-sections of the fiber, which has repercussion in the amplification efficiency. Thus, a fiber with a higher P concentration will exhibit lower cross-sections, therefore requiring a higher Yb-ion concentration to reach a certain desired amplification efficiency. However, increasing the Yb-ion concentration leads to higher photodarkening losses, which might potentially counteract the benefits of using P co-doping. In this paper we present a comparative analysis of the expected performance of different fiber amplifiers for a given constant average heat-load and amplification efficiency as a function of the ratio of Al:P concentration in the fiber core. This study indicates which core compositions are more beneficial for increasing the mode instability threshold in Yb-doped high-power fiber amplifier systems.
B. Lei, J. Wang, V. Kharin, M. Zepf, and S. Rykovanov
γ-Ray Generation from Plasma Wakefield Resonant Wiggler
Phys. Rev. Lett., 120 :134801 (March 2018)
A flexible gamma-ray radiation source based on the resonant laser-plasma wakefield wiggler is proposed. The wiggler is achieved by inducing centroid oscillations of a short laser pulse in a plasma channel. Electrons (self-)injected in such a wakefield experience both oscillations due to the transverse electric fields and energy gain due to the longitudinal electric field. The oscillations are significantly enhanced when the laser pulse centroid oscillations are in resonance with the electron betatron oscillations, extending the radiation spectrum to the gamma-ray range. The polarization of the radiation can be easily controlled by adjusting the injection of the laser pulse into the plasma channel.
U. Zastrau, C. Rödel, M. Nakatsutsumi, T. Feigl, K. Appel, B. Chen, T. Döppner, T. Fennel, T. Fiedler, L. B. Fletcher, E. Förster, E. Gamboa, D. O. Gericke, S. Göde, C. Grote-Fortmann, V. Hilbert, L. Kazak, T. Laarmann, H. J. Lee, P. Mabey, F. Martinez, K.-H. Meiwes-Broer, H. Pauer, M. Perske, A. Przystawik, S. Roling, S. Skruszewicz, M. Shihab, J. Tiggesbäumker, S. Toleikis, M. Wünsche, H. Zacharias, S. H. Glenzer, and G. Gregori
A sensitive EUV Schwarzschild microscope for plasma studies with sub-micrometer resolution
Rev. Sci. Instrum., 89 :023703 (February 2018)
We present an extreme ultraviolet (EUV) microscope using a Schwarzschild objective which is optimized for single-shot sub-micrometer imaging of laser-plasma targets. The microscope has been designed and constructed for imaging the scattering from an EUV-heated solid-density hydrogen jet. Imaging of a cryogenic hydrogen target was demonstrated using single pulses of the free-electron laser in Hamburg (FLASH) free-electron laser at a wavelength of 13.5 nm. In a single exposure, we observe a hydrogen jet with ice fragments with a spatial resolution in the sub-micrometer range. In situ EUV imaging is expected to enable novel experimental capabilities for warm dense matter studies of micrometer-sized samples in laser-plasma experiments.
H. Gies, F. Karbstein, and C. Kohlfürst
All-optical signatures of strong-field QED in the vacuum emission picture
Phys. Rev. D, 97 :036022 (February 2018)
We study all-optical signatures of the effective nonlinear couplings among electromagnetic fields in the quantum vacuum, using the collision of two focused high-intensity laser pulses as an example. The experimental signatures of quantum vacuum nonlinearities are encoded in signal photons, whose kinematic and polarization properties differ from the photons constituting the macroscopic laser fields. We implement an efficient numerical algorithm allowing for the theoretical investigation of such signatures in realistic field configurations accessible in experiment. This algorithm is based on a vacuum emission scheme and can readily be adapted to the collision of more laser beams or further involved field configurations. We solve the case of two colliding pulses in full 3+1-dimensional spacetime and identify experimental geometries and parameter regimes with improved signal-to-noise ratios.
M. Chemnitz, C. Gaida, M. Gebhardt, F. Stutzki, J. Kobelke, A. Tünnermann, J. Limpert, and M. A. Schmidt
Carbon chloride-core fibers for soliton mediated supercontinuum generation
Opt. Express, 26 :3221 (February 2018)
We report on soliton-fission mediated infrared supercontinuum generation in liquid-core step-index fibers using highly transparent carbon chlorides (CCl4, C2Cl4). By developing models for the refractive index dispersions and nonlinear response functions, dispersion engineering and pumping with an ultrafast thulium fiber laser (300 fs) at 1.92 µm, distinct soliton fission and dispersive wave generation was observed, particularly in the case of tetrachloroethylene (C2Cl4). The measured results match simulations of both the generalized and a hybrid nonlinear Schrödinger equation, with the latter resembling the characteristics of non-instantaneous medium via a static potential term and representing a simulation tool with substantially reduced complexity. We show that C2Cl4 has the potential for observing non-instantaneous soliton dynamics along meters of liquid-core fiber opening a feasible route for directly observing hybrid soliton dynamics.
J. H. Bin, M. Yeung, Z. Gong, H. Y. Wang, C. Kreuzer, M. L. Zhou, M. J. V. Streeter, P. S. Foster, S. Cousens, B. Dromey, J. Meyer-ter-Vehn, M. Zepf, and J. Schreiber
Enhanced Laser-Driven Ion Acceleration by Superponderomotive Electrons Generated from Near-Critical-Density Plasma
Phys. Rev. Lett., 120 :074801 (February 2018)
We report on the experimental studies of laser driven ion acceleration from a double-layer target where a near-critical density target with a few-micron thickness is coated in front of a nanometer-thin diamondlike carbon foil. A significant enhancement of proton maximum energies from 12 to ∼30  MeV is observed when a relativistic laser pulse impinges on the double-layer target under linear polarization. We attributed the enhanced acceleration to superponderomotive electrons that were simultaneously measured in the experiments with energies far beyond the free-electron ponderomotive limit. Our interpretation is supported by two-dimensional simulation results.